The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates many of the biological and toxicological actions of a variety Of hydrophobic natural and synthetic chemicals, including the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin. The best characterized high affinity AhR ligands include a variety of toxic and carcinogenic halogenated and polycyclic aromatic hydrocarbons; no high affinity endogenous physiological AhR ligand has been identified. Studies in our laboratory have indicated that the AhR can be activated by a variety of structurally dissimilar chemicals and have revealed the existence Of species differences in AhR ligand binding specificity. These results not only suggest that the AhR has a very promiscuous ligand binding site, but they raise significant questions as to the spectrum of chemicals which can bind and activate the AhR in different species. Knowledge of the diversity of AhR ligand structure may provide insight into the identity of its physiological ligand(s). We hypothesize that the AhR can bind and be activated by a wide variety of structurally dissimilar endogenous and exogenous chemicals and that the activation of the AhR is a ligand-dependent event which exhibits species specificity. The overall goals of this proposal are to further characterize the ligand binding specificity of the AhR from different species in order to both confirm the ligand-dependent nature of AhR activation and to identify novel endogenous and exogenous ligands/activators of the AhR signal transduction pathway. The mechanism by which benzimidazoles, selected pesticides and other chemicals activate the AhR in a "ligand-independent manner" and the ability of some endogenous physiological chemicals, namely arachidonic acid-derived lipids and tryptophan/indole-derived products, to activate/augment AhR-dependent signaling will be examined using ligand binding and DNA binding and gene expression assays. Species- specific differences in ligand-dependent activation of the AhR will be assessed using ligand and DNA binding assays and in cells stably transfected with species-specific AhR cDNAs and an AhR-responsive luciferase reporter plasmid. AhR-based bioassays will also be used in combination with analytical chemical fractionation procedures to purify and elucidate the chemical structure of novel AhR ligands/activators we have identified in water and ethanol extracts of commercial products. Overall, these studies will define the spectrum of chemicals which can bind to and activate the AhR in various species and provide insights into the identity of the endogenous physiological AhR ligand(s) and the role of this receptor in normal physiological processes.